There's always a chance they may not be aware. Perhaps someone could leave a polite note on their car informing them of the fact it's a private drive they're allegedly parked on, that the owners can't gain access to their property. That way if they do it again then stick a notice of intent saying the next time they do it their car will be legally towed, at their expense.
Parking on our private drive and blocking us from getting in to our own house